1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copy machine, a printer, or a facsimile machine. More specifically, the present invention relates to a configuration of a fixing device.
2. Description of the Related Art
A fixing device is known that includes cylindrical metal heat-conductive member and a heat source that are provided inside an endless belt that is heated via the metal heat-conductive member and further includes a pressure roller that makes a contact with the metal heat conductive member with the endless belt in between, thereby forming a nip part, in which the endless belt is caused to runs in the circumferential direction in accordance with the rotation of the pressure roller (see, for example, Japanese Laid-open Patent Publication No. 2007-334205). This configuration makes it possible to heat the whole endless belt that configures the fixing device, shorten the first print time from the heating stand-by time, and overcome the heat shortage upon high-speed rotation.
Another fixing device is known in which only a member that forms a nip, a member that supports the member, and a heart source that are provided inside an endless belt and thus the endless belt can be directly heated with the heat source to form a nip (see, for example, Japanese Laid-open Patent Publication No. 2007-233011). This configuration reduces the heat capacity around the heat source, thereby shortens the first print time from the heating stand-by time.
In the conventional configurations, a supporting member supporting a nip member has to penetrate through the inside of the belt and thus the supporting member is a double-supported beam that bears the load from the nip surface at both ends. In order to reduce the size of the fixing device and the extra heat capacity, it is preferable that the volume of the supporting member be small as possible; however, because the supporting member has to form a nip width while bearing the load applied from an elastic pressure roller, if the volume of the supporting member is reduced and accordingly the strength lowers, the center of the supporting member bends in the direction away from the nip surface and accordingly a uniform nip width in the longitudinal direction cannot be obtained.
As a method that solves this problem, a method can be assumed in which the center of a nip forming member or a supporting member in the longitudinal direction is formed convex against the nip surface to cancel the bend; however, it is required to form the convex part in an accuracy of 0.05 mm or smaller in order to obtain an even nip surface and thus it costs a lot due to the total inspection etc.
In view of the above-described conventional problems, there is a need to provide a fixing device that solves the above-described problem and increases the reliability of the parts with a configuration that allows forming of an accurate convex part relatively easily, which makes it possible to reduce the inspection costs, etc.